The present invention relates generally to the overall structuring of heat exchangers and more particularly to a structure for a heat exchanger assembly of the counterflow type having a longitudinal configuration. More specifically, the invention relates to the type of heat exchanger assembly wherein groups of tubes are arranged to form a plurality of nests of such tubes, with the individual tubes of each nest being connected at their ends with a tube plate and with each of the nests surrounded by a guide shell which permits a heat exchange medium flowing around the outside of the tubes to be introduced and exhausted from each of the tube nests.